The present invention relates to a voltage controlled oscillator including an oscillator circuit with a control input, an output terminal and a variable capacitor, said oscillator circuit being adapted to provide, at said output terminal, an output signal having a frequency that is a function of the value of said variable capacitor, said value being controlled by a control voltage applied to said control input.
Such a voltage controlled oscillator is already known in the art. It generally forms part of a Phase Locked Loop [PLL] arrangement wherein the frequency of the output signal has to be selected within a relatively large frequency range. This is for instance the case when the voltage controlled oscillator is used in mobile telecommunication systems and if different transmission protocols, such as the known Global System for Mobile communications [GSM] at 900 MHz and Digital Enhanced Cordless Telecommunications [DECT] or Digital Crossconnect System [DCS] in the 1,800 MHz frequency band, have to be covered by a same circuit. Since the frequency is dependent of the control voltage, the latter must then also be able to vary within a relatively large voltage range.
Furthermore, due to variations in the processing technology, the frequency of the output signal may shift, for a same control or tuning voltage, from the desired one by a large amount. Here again, a solution of using a large tuning range requires a high supply voltage and/or an abrupt variable capacitor or xe2x80x9cvaricapxe2x80x9d. A large tuning range results in a large sensitivity of the VCO, which severely influences the surrounding parts of the PLL arrangement.
The trend is now however to reduce the supply voltage in order to reduce accordingly the production cost. Low supply voltage means a reduction of the number of batteries and allows using xe2x80x9cdeep submicron technologyxe2x80x9d that permits to reduce the production cost but that can not accept high supply voltages because of risks of break-down.
A problem rising with the reduction of the supply voltage is that a single voltage controlled oscillator may then no longer reach the limits of the requested frequency range.
A possible solution should be to integrate two VCOs in a same circuit. However, such a solution is avoided because of the relatively high cost of this component.
Another possible solution is to design and fabricate several VCOs with xe2x80x9ccenterxe2x80x9d frequencies shifted by a certain amount. A center frequency is the frequency at which the VCO should normally operate and that preferably corresponds to a control voltage having an average value, i.e. in the middle of the supply voltage range. Only the VCOs having the correct frequency, under the actual technology characteristics, should then be selected and activated. This also results in an expensive and non-efficient process.
It is finally also possible to perform a digital calibration of the center frequency with a factory trimming. However, digital factory calibration is costly and does not cover variations over the products life time.
An object of the present invention is to provide a voltage controlled oscillator of the known type but adapted to operate with a relatively low power supply, while allowing a large operational frequency range and keeping the production cost low.
According to the invention, this object is achieved due to the fact that said voltage control oscillator further includes calibration means and switchable capacitor means which comprises switches and capacitors, said calibration means comprising comparator means coupled to a detection circuit, that said comparator means has a first input to which said control voltage is applied, at least a second input to which a predetermined reference voltage is applied, and an output connected to a level input of said detection circuit, that said detection circuit has a control output coupled to switch control inputs of the switches of said switchable capacitor means, said switches being adapted to connect/disconnect, in parallel to said variable capacitor, a capacitor of said switchable capacitor means, that said detection circuit is adapted to activate a control signal if a signal received at said level input indicates that said control voltage has reached said predetermined reference voltage, and that said detection circuit is further adapted to provide said activated control signal at said control output in order to control the switches of said switchable capacitor means.
In this way, when the control voltage reaches the predetermined reference voltage, the fact of adding or removing a capacitor in parallel with the variable capacitor of the oscillator circuit makes the frequency to jump towards another control voltage versus frequency characteristic. As a result, in function of the number of capacitors branched in parallel with the variable capacitor of the oscillator circuit, different frequencies may be obtained for a same control voltage. This allows to increasing the frequency range of a single VCO at relatively low cost and is especially useful in low power supply applications.
Another characterizing embodiment of the present invention is that said voltage control oscillator further includes a feedback circuit having a feedback input to which said output terminal is connected and a feedback output at which said feedback circuit is adapted to provide said control voltage, that said feedback circuit further has a feedback control input to which a reset output of said detection circuit is connected, said detection circuit being adapted to apply a reset pulse to said reset output when said control signal is activated, and that said feedback circuit is adapted to provide, at said feedback output, said control voltage as a function of the frequency of a signal at said feedback input when no reset pulse is received from said detection circuit, and to provide said control voltage equal to a predetermined middle reference voltage when a reset pulse is received from said detection circuit.
When the control voltage changes, say increases, the frequency of the output signal also increases according to a control voltage versus frequency characteristic of the VCO. When the control voltage reaches the predetermined reference voltage, the control signal is activated by the detection circuit and causes the frequency to jump to another control voltage versus frequency characteristic, i.e. to another frequency value. This new frequency value is then generally higher than the requested frequency. This is for instance the case where the control voltage is used for fine-tuning purposes. As a consequence, the control voltage, being then anyway to high and not allowed increasing anymore, needs to be decreased to obtain the correct frequency value. The detection circuit applying a reset pulse to the reset output when the control signal is activated performs this. This forces the feedback circuit to provide a control voltage equal to the predetermined middle reference voltage. This new control voltage is lower than the previous one so that it may again increase thereby increasing the frequency of the output signal.
Preferably, said predetermined middle reference voltage has an intermediate value between said predetermined low reference voltage and said predetermined high reference voltage.
The predetermined middle reference voltage is thus generally a voltage in the middle of the supply voltage range. This allows the frequency to be changed in both directions, i.e. up or down, starting from an average value.
Also another characterizing embodiment of the present invention is that said voltage control oscillator further includes selection means having a first selection input to which said control voltage is applied, a second selection input to which said predetermined middle reference voltage is applied, a selection output connected to the control input of said oscillator circuit, and a selection control input to which the reset output of said detection circuit is connected, and that said selection means is adapted to connect said first selection input to said selection output if no reset pulse is received at said selection control input, and to connect said second selection input to said selection output if said reset pulse is received.
This improves the speed of the transfer from one characteristic to another with a reduced frequency jump during the change of the control voltage from the predetermined reference voltage to the predetermined middle reference voltage by the feedback circuit under control of the activated control signal. This is also useful during a startup phase of the VCO. The VCO then starts operating with a control voltage in the middle of the supply voltage range. This is called xe2x80x9cautomatic center frequency calibrationxe2x80x9d and allows the control voltage to be modified for a same amount up and down from the middle or center frequency.
The present invention also relates to a method to adjust a control voltage of a voltage controlled oscillator wherein the frequency of an output signal is a function of said control voltage, said method comprising the step of applying said control voltage to a variable capacitor of an oscillator circuit.
Such a method is already known in the art and is for instance used in the known VCOs mentioned above. If the method is so used, the same problems as described above will arise and no suitable solution is known.
Another object of the present invention is to provide a method adapted to operate a VCO with a relatively low power supply, while allowing a large operational frequency range and keeping the production cost low.
According to the invention, this object is achieved due to the fact that the present method further comprises steps of comparing said control voltage with a predetermined reference voltage, and of controlling the coupling of a capacitor in parallel to said variable capacitor when said control voltage reaches said predetermined reference voltage.
In this way, when the control voltage reaches the predetermined reference voltage, the fact of coupling, i.e. adding or removing, a capacitor in parallel with the variable capacitor of the oscillator circuit makes the frequency to jump to another value. As a result, different frequencies may be obtained for a same control voltage and a large frequency range may be achieved with a reduced voltage range for the control voltage.
Another characterizing embodiment of the present invention is that the method further comprises a step of resetting said control voltage to a predetermined middle reference voltage when said predetermined reference voltage is reached.
When the control voltage changes, say decreases, until reaching the predetermined reference voltage, the frequency jumps to another value. This new frequency value is lower but corresponds to a control voltage that is not allowed to decrease anymore. By resetting the control voltage to the predetermined middle reference voltage which is preferably a voltage in the middle of the supply voltage range, and thus lower than the actual control voltage, the latter may again decrease, thereby allowing again the frequency to be modified in any direction, i.e. higher or lower.
It is to be noted that the present method is particularly suited to be applied in the above VCO according to the invention.
Further characterizing embodiments of the present voltage controlled oscillator, preferably with an automatic center frequency calibration as mentioned above, and of the method to adjust the control voltage thereof are mentioned in the appended claims.
It is to be noticed that the term xe2x80x98comprisingxe2x80x99, used in the claims, should not be interpreted as being limitative to the means listed thereafter. Thus, the scope of the expression xe2x80x98a device comprising means A and Bxe2x80x99 should not be limited to devices consisting only of components A and B. It means that with respect to the present invention, the only relevant components of the device are A and B.
Similarly, it is to be noticed that the term xe2x80x98coupledxe2x80x99, also used in the claims, should not be interpreted as being limitative to direct connections only. Thus, the scope of the expression xe2x80x98a device A coupled to a device Bxe2x80x99 should not be limited to devices or systems wherein an output of device A is directly connected to an input of device B. It means that there exists a path between an output of A and an input of B which may be a path including other devices or means.